Project Summary We seek support to develop and build the positron emission tomography (PET) detector module for the next generation 7-Tesla magnetic resonance (MR)-compatible PET brain scanner with dramatically improved spatio-temporal resolution. PET and MRI are two of the most powerful imaging modalities currently in use for studying the human brain. Recently, scanners capable of simultaneous PET and MR whole-body data acquisition in human subjects have become commercially available. However, there is no equivalent dedicated head device on the market to address the needs of the researchers and clinicians focusing on the brain and the performance of whole-body devices is rather limited for this purpose. More importantly, although current PET technology achieves high molecular sensitivity with a broad set of probes for neurochemical targets, PET still lacks the capability to track dynamic changes in a time scale comparable to functional processes. Our long-term goal is to build an MR-compatible PET camera with very high sensitivity to enable truly dynamic PET imaging of brain neurotransmission. One of the first MR-compatible brain PET prototypes was installed at the Martinos Center in 2008 when human PET/MR imaging was in its infancy. Following a close collaboration with Siemens to address the remaining technical challenges, proof-of-principle PET/MR studies demonstrating the advantages and potential of this novel imaging modality were performed. Almost a decade later, a new type of photon detector technology has reached a level of maturity that would allow us to build the next generation integrated system with dramatically improved spatiotemporal resolution. As first steps towards achieving this goal, we propose to design, build and evaluate a high performance PET detector module and begin to address the system-level integration challenges. Specifically, we will: (1) develop a depth-of-interaction and time-of- flight capable scintillator array using a practical approach; (2) design, build and evaluate a scalable 7-T MR- compatible silicon photomultiplier based PET detector module; and (3) investigate and address the system- level hardware challenges for a non-standard PET scanner geometry. !